Some interesting details from the InfoPath 2010 Capacity Planning White Paper

 

• If a scenario involves saving a form to a document library, it is better to submit the form to the library instead of saving it. A Submit operation triggers only one POST request or round trip, whereas a Save operation triggers two POST requests. The name of the form can be dynamically generated by using a rule or by using a control in the form.
• Document library forms can achieve greater throughput than InfoPath list forms. If high throughput is needed for a solution, consider using a document library form instead of an InfoPath list form.

 

Read the rest (and a lot of other fantastic SP2010 service capacity docs at https://www.microsoft.com/downloads/en/details.aspx?FamilyID=fd1eac86-ad47-4865-9378-80040d08ac55&displaylang=en